The present invention relates to a method of making a polymer-metal compound whose electrical conductivity varies within large limits and can be adjusted to a desired value and, more particularly, to a method of making an electrically-conductive polymer-metal compound of a predetermined conductivity on a substrate made of inorganic or organic material.
Methods of making an electrically conductive polymer-metal compound are known, in which methods volatile metallo-organic compounds are decomposed in a glow discharge zone in a reactor and are polymerized into the polymer-metal compound containing the metallic components. These methods are known as plasma polymerization methods.
A method of plasma polymerization of iron carbonyl and related iron metallo-organic compounds has been investigated by Sadhir and Saunders. This method is described in the article "Deposition of conducting thin films of organometallic monomers by plasma polymerization" by R. K. Sadhir and H. E. Saunders in J.Vac.Sci. Technol. A 3(6), 1985, pp. 2093-2097. In this method no additional metal-free organic compounds are used in the reactant mixture in which the glow discharge is formed so that an additional parameter effecting the ratio of metal to carbon in the discharge is not available. There is no teaching of a way to control the deposition so that a coating with a predetermined electrical conductivity can be obtained. The value of the conductivity of the conductive film which is produced depends on the geometry of the apparatus used to produce the film, especially the distance between the supply location of the metallo-organic compound and the substrate. It is understandable that a polymer-metal film with a predetermined desired resistance value cannot be produced by this method.
A method of producing conductive metal-fluorocarbon polymer films has been described by Perrin, Despax, Hanchett and Kay in the article, "Microstructure and electrical conductivity of plasma deposited gold/fluorocarbon composite film" by J. Perrin, B. Despax, V. Hanchett and E. Kay, J.Vac.Sci.Technol. A 4(1), 1986, pp. 46 to 51. The substrate is exposed to a plasma discharge in a fluorocarbon and argon gas mixture in this method. Instead of using a gaseous metallo-organic compound as a source of metal atoms for the plasma produced in the fluorocarbons gold or other metal atoms are added to the composite film produced on the substrate by RF sputtering methods using a gold coated electrode. The method produces an organic polymer film, in which metallic particles in the form of numerous small spots spaced from each other are embedded. These films must contain at least about 50% metal atoms to be conductive. Typical properties of the films produced are the limited values of "spot density" and the 50% threshold for film conductivity. Thus, it is almost impossible to obtain a film with an electrical conductivity above a certain value.
Morosoff and Haque have studied the plasma polymerization of volatile transition metal compounds and C.sub.2 hydrocarbons. A coating was formed by exposing a substrate to a plasma made in a reactant gas containing an iron carbonyl and a C.sub.2 hydrocarbon. The nature of the products and chemical processes occurring during film formation was determined. No measurements of electrical conductivity were made however and there was no study of the effect of varying process parameters on the electrical conductivity of the film formed, so that there is no way to know if an electrically conductive polymer layer would form under the conditions used in their experiments. Similarly there is no suggestion that their method could be used to produce an electrically conductive layer. This work was described in the article "Transition metal containing plasma polymers" by N. Morosoff and R. Haque and S. D. Clymer and A. L. Crumbliss in J.Vac.Sci.Technol. A 3(6), 1985, pp.2096 to 2101.